What I was trying to do: I tried to insert a command (xbacklight -set 50) that sets my monitor brightness at the lightdm config file. Now when I start my laptop, it gets stuck at the xubuntu loading screen, so I can't login.

Is there a way to go into the shell/command-line and fix the lightdm.conf file??

Solution: CTRL + ALT + F1 at the xubuntu loading screen. Then gained access to the shell, used vim to edit the file and then removed the commands I had inserted on it that was preventing lightdm from starting..